Reflector antenna with a self-supported feed

ABSTRACT

A reflector antenna with a dish-shaped main reflector (10), and a self-supporting feed (11) for the transmission or reception of polarized electromagnetic waves. The feed (11) consists of a tube (12) which is attached to the middle of the main reflector (10) and is terminated by a subreflector (13) so that an intermediate space (14) is formed between the subreflector and the end of the tube. The part of the tube that is nearest the intermediate space (14) contains a cylindrical waveguide (15), or is the waveguide itself, and has an approximately circular or quadradic cross-section. Externally, the intermediate space (14) is bonded by a circular, cylindrical surface (16) with the same diameter as the outer diameter of the tube (12) this being called the aperture surface. The surface of the subreflector (13) which is located just outside the surface of the aperture (16) has circular corrugations (17), or other means of creating a reactive, anisotropic surface impedance, to ensure that the electromagnetic waves are propagated along the surface regardless of whether the electrical field is tangential to the surface or is normally on it. The part of the subreflector (13) that is located within the aperture surface (16) is shaped as a central conical element (18) with reflecting characteristics and which is inclined towards the tube (12).

FIELD OF THE INVENTION

The invention consists of a reflector antenna with a self-supported feedfor the transmission or reception of polarized electromagnetic waves.The antenna is principally intended for the reception of TV signals fromsattelites, however it can be used as a radio link, and as a groundstation for sattelite communications.

BACKGROUND OF THE INVENTION

These types of reflector antennas are chiefly used because they arestraightforward and inexpensive to manufacture. They also providegreater antenna efficiency and lower side lobes in the radiaton patternthan is the case when the feed element has to be supported by diagonalstruts. The drawback with the latter configuration is that the mainreflector becomes blocked. A self-supported feed is also easelyaccessible from the back of the reflector, thus is frequently selectedwhen it is best to locate the transmitter and/or the receiver there.This also reduces the loss that occurs when the waves have to be led ina cable along one at the support struts.

A. Chlavin, "A New Antenna Feed Having Equal E and H-Plane Patterns",IRE Trans. Antennas Propagat., Vol. AP-2, pp. 113-119, Jul. 1954,describes a reflector antenna with a self-supporting feed. However sincethis antenna uses a waveguide with a rectanguler cross-section, it canonly transmit or receive waves with one particular linear polarization.

C. C-Cutler, "Parabolic-antenna design for microwaves", Proc. IRE, Vol.35, pp. 1284-1294, Nov. 1947, describes a dual polarized reflectorantenna with two variants of a self-supporting feed, called the "ringfocus" and the "waveguide cup" feeds respectively. A circular waveguideis used in these two feeds with a reflecting object in front of thewaveguide opening, this reflector being respectively shaped like a flatdisc and a cup. Both of these feeds unfortunately produce highcross-polarization within the main lobe the radiation pattern.

SUMMARY OF THE INVENTION

The main purpose of the present invention is to design a reflectorantenna which has dual polarization with low cross-polarization withinthe main lobe of the radiation pattern. Dual polarization means that theantenna is capable of receiving or transmitting two waves withorthogonal linear or circular polarization simultaneously. The waveguidemust have an almost circular or quadratic cross-section.

This objective can be achieved by a design which is in accordance withthe charaterizing part of Claim 1. Further details about the inventionare given in Claims 2-10.

The surface of the subreflector is treated so that the electromagneticwaves are reflected from and propagate along the surface inapproximately the same way regardless of whether the electric field isnormally on the surface or is tangential to it. Furthermore, the designof the other geometries of the feed ensures that the cross-polarizationremains low within the main lobe of the radiaton pattern.

It should be mentioned that a dual polarized reflector antenna with aself-supporting feed is already known from among other sources such asP. Newham, "The Search for an Efficient Splashplate Feed", Proceedingsof the Third International Conference on Antennas and Propagation (ICAP83), IEE Conference Publication No. 219, pp. 348-352, Apr. 1983, and inprevious publications by the same author. In this design thesubreflector has a smooth surface. However, it is also possible toobtain low cross-polarization when the subreflector is positioned at adistance from the waveguide aperture so that the waves are preventedfrom becoming radial and cannot propagate along the surface of thesubreflector. This avoids the polarization-dependent reflectioncoefficient for radial waves found in the smooth subreflector. Thepresent invention, on the other hand, has conceived of an antenna wherethis distance is so small that some of the waves are able to propagatealong the surface of the subreflector. Low cross-polarization is thenonly ensured by a surface where the reflection coefficient for radialwaves is independent of the polarization.

The main advantage of the present invention over P. Newham's solution isthat the diameter of the subreflector can be reduced so that theblockage in the center of the main reflector is also smaller.

It should also be noted that a dual polarized antenna that radiatesaround a cylinder is described by A. W. Love, "Scale Model Developmentof a High Efficency Dual Polarized Line Feed for the Arecibo SphericalReflector", IEEE Trans. Antennas Propagat., Vol. AP-21, pp. 628-639,Sept. 1973. This antenna is, however, a linear array antenna consistingof numerous elements, which feed a main spherical reflector antenna.Further, this antenna has no subreflector.

Mention should also be made of a dual polarized element which radiatesaround a smooth conductor cylinder. This is reported by P. S. Kildal in"Study of Element Patterns and Excitations of the Line Feed of theSpherical Reflector Antenna in Arecibo". IEEE Trans. Antennas Propagat.,Vol. AP-34, pp. 197-207, Feb. 1986. Section 11 of this paper provides atheoretical analysis of such an element. Once again there is nosubreflector, and the element does not feed a main reflector. One resultof this theoretical work is in fact the present invention.

In U.S. Pat. No. 3,162,858 a dual polarized reflector antenna isdescribed with a self-supporting feed element which mainly consists of aradial waveguide shaped as two plane surfaces or two coaxial conicalsurfaces with a common apex. In the present invention there are no suchradial waveguides; a subreflector is employed instead.

Since the tube in the present invention is cylindrical rather thanconical, the subreflector and the outside of the tube are unable to formradial waveguides. Consequently, the waves are not propagated in theform of radial wave modes in this area, as is the case in the U.S.Patent mentioned above.

The U.S. Patent describes an antenna with a ring-shaped focus (theequivalent to the phase center of the feed element) in the opening oraperture of the radial waveguide, and there is no subreflector outsidethis phase-center. In the invention however, the feed elementring-shaped phase center is close to the cylindrically-shaped aperturesurface between the end of the tube and the middle of the subreflector.Consequently, in the invention the subreflector is mainly outside thephase center.

In the U.S. Patent both walls in the radial waveguide have circularcorrugations which are approximately 0.25λ wavelengths deep. Thesecorrugations give the walls an anisotropic surface impedance whichresults in the radial waves being propagated so that they areindependent of the polarization in the waveguide. In the presentinvention. It is first and foremost only the subreflector which issupplied with such an anisotropic, reactive surface impedance. Using theinvestigations derived from the formulae in the paper already mentionedin IEEE Trans. Antennas Propagat., Vol. AP-34, Feb. 1986, it has beenfound that in most cases it is unnecessary to treat the outside of thetube with such a surface impedance. This consequently makes theinvention cheaper to manufacture than the existing antenna where twosurfaces have to be corrugated.

There is no reason why the outside of the tube described in the presentinvention cannot be given an anisotropic reactive surface impedance,this may even be advantageous since in some applications particularlystrict demands regarding cross-polarization may be required.

The present invention is based on a theoretical model concerning the waywhich radiation is released from a circumterencial slot in acylinderical tube (cf. the paper mentioned in IEEE Trans. Antennas andPropagat., Vol. AP-34, Feb. 1986).

The bandwidth problem in the invention is solved by the central part ofthe subreflector being designed as a cone that is aimed in the directionof the main reflector. This cone reflects the incidence waves from thewaveguide in a radial direction so that only small amplitude waves arereflected back to the waveguide. This minimizes return loss. At the sametime a correct balance is achieved between the axial and thecircumferential E-fields over the cylindrical aperture, thus ensuringlow cross-polarization. This can be achieved over a relative bandwidthof about 10%.

All mechanical dimensions between the middle of the subreflector and theend of the tube are critical, nevertheless there are a good number ofdimension combinations which provide satisfactory results.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained in more detail by making reference tothe drawings, where:

FIG. 1 illustrates an example of a reflector antenna with aself-supporting feed,

FIG. 2 shows an axial cross-section through a feed designed inaccordance with the invention,

FIG. 3 shows an axial cross-section through a subreflector which has acorrugated surface,

FIG. 4 shows an axial cross-section through a tube with circularcorrugations on the surface,

FIG. 5 shows a normal section on a tube with longitudinal corrugationson the surface,

FIG. 6 shows an axial cross-section through a means of designing a feedelement in accordance with the invention, and,

FIG. 7 indicates which dimensions for the design in FIG. 6 must betrimmed and are critical.

DETAILED DESCRIPTION OF THE INVENTION

The antenna in FIG. 1 consists of a dish-shaped main reflector 10. Inthe middle of the reflector there is a self-supporting tubular feedelement 11. This consists of a cylindrical tube 12, and a subreflector13. The tube and the subreflector are separated by a gap 14 which isbounded on the outside by a circular, cylindrical aperture surface 16which will henceforth be termed the aperture surface or the aperture.

FIG. 2 shows an axial section through the feed. The tube 12 contains acylindrical waveguide 15 which preferably has a circular cross-section.The tube can also be such a waveguide itself. The waveguide isconstructed to propagate the basic mode. This is the TE₁₁ mode when theinternal cross-section is circular with smooth, conducting walls. Thewaveguide must have a larger diameter than 0,6 (approx.) wavelengths λand be smaller than 1.2λ (approx.). The tube and the waveguide aremostly made of conducting materials. Though a smooth surface is shown,it could also be manufactured so that the surface impedance isanisotropic and reactive. The thickness of the walls measured betweenthe inside of the waveguide and the outside of the tube is less than1.0λ (approx.). The wall can also be extremely thin. FIG. 2 shows a casewhere the gap 14 extends slightly into the tube so that a circularwaveguide is formed with a larger diameter than waveguide 15. The gap 14can also have another design.

The subreflector is drawn as a plate with a conical element 20 in themiddle. It can also be shaped otherwise. The part of the subreflector'ssurface that is located outside the aperture surface 16 is drawn toappear smooth, however in fact it is treated so that the surfaceimpedance is anisotropic and reactive. This ensures that theelectromagnetic waves are reflected from and propagate along the surfacein approximately the same way regardless of whether the electric fieldis normally on the surface or is tangential to it. This is important toachieve low cross-polarization. The best results come from making thesurface impedance so that there is only a minor amount of radiation in aradial direction along the subreflector both when the field is normallyon the surface and when it is tangential to it. The diameter of thesubreflector is always larger than the diameter of the tube, typicalvalues are between 3λ and 6λ.

The aperture surface 16 is indicated in FIG. 2 by a broken line. Thecross-section of the aperture 16 is under 1.0, λ preferably 0,5λ(approx.). In the same way the end of the waveguide 15 is marked by abroken line. There is a gap 14 between the aperture and the end of thewaveguide, this is bounded by the subreflector and the tube. The gap 14is drawn so that it appears to be filled be air. In practice they wouldbe partly or totally filled with dielectric matter, or they could bepartially sealed with metallic or dielectric rods or discs that arerespectively located in a plane with the axis of symmetry. Though thisis necessary to attach the subreflector to the tube, this is also ameans of controlling the excitation of the two modes over the aperture16 and hence the radiation characteristics.

FIG. 3 shows an axial cross-section of a subreflector 13 where the partthat lies outside the aperture 16 has circular corrugations or grooves17 in the surface. This grooves are about 0,25λ deep. This is one way ofrealizing the anisotropic and reactive surface impedance. The objectiveis as mentioned before to obtain as little radiation as possible in aradial direction along the subreflector both when the field is normallyon the surface and also when it is tangential to it. This is importantto obtain low cross-polarization. This objective can also be achieved bya surface having other characteristics.

FIG. 4 shows an axial cross-section of a tube 12 where there arecircular corrugations 18 in the surface. These corrugations are about0,25λ deep and produce an anisotropic, reactive surface impedance. Thepurpose is to obtain as little radiation as possible along the tube bothwhen the field is orthogonal to the surface and when it is tangential toit. This can also be achieved by a surface with differentcharacteristics.

FIG. 5 shows a cross-section of a tube 12 where the surface haslongitudinal corrugations 19. These are filled with a dielectric havinga relative permittivity of ε. The depth of the corrugations 0,25λ/ε-1.These corrugations provide an anistropic, reactive surface impedance.The objective is to produce powerful radiation along the tube both whenthe field is normally on the surface and when it is tangential to it.This can also be managed by using a surface having othercharacteristics.

FIG. 6 shows a normal means of designing the feed element. The gap 14 isfilled with a dielectric plug or element 21 which is glued or screwedinto both the tube and the subreflector by means of an extra groove 23inside the aperture surface or by means of a central outlet 22 in theconical part 20 of the subreflector 13. The part of the subreflector 13which lies outside the aperture surface is plane and has circularcorrugations. The dielectric plug 21 passes into the tube 12 and forms acylindrical waveguide with a larger diameter than the waveguide 15. FIG.7 also shows the design in FIG. 6. The critical dimensions which must betrimmed in the laboratory model are marked x, y, z and 2a. This can bedone by making the concial element 20 so that it can be screwed into thesubreflector. In addition, the waveguide 15 and the dielectric plug 21are both to be made so that they can be screwed into the tube 12. Themanner in which the design in FIG. 6 works for linear polarization isexplained in the next paragraph. In the case circular polarization thedesign works in an equivalent way because the geometry has rotationalsymmetry. The manner of operation is explained for transmission, but isequivalent when receiving.

A wave in the TE₁₁ mode is propagated in the waveguide 15. This wave iscoupled to two modes at the surface of the aperture 16. For one mode theelectric fields are directed exclusively in the z-direction (z-mode),and for the other the fields are directed in the azimuth-directiontransverse to the z-direction (φ-mode). These two modes radiate out ofthe aperture 16, the z-mode principally in the E-plane and the φ-modechiefly in the H-plan. To get a rotationally-symmetrical radiationpattern with low cross-polarization, the radiation patterns in the E andH-planes must be similar in both amplitude and phase. The anisotropicand reactive surface impedance to the subreflector 13 is the reason whythe z-mode radiates the same way in the E-plane as the φ-mode radiatesin the H-plane. At the same time the internal dimensions of the feedelement are controlled so that the z-mode and the φ-mode are excited bythe correct amplitude and phase, relatively-speaking. The z-mode and theφ-mode radiate differently along the tube. This can be improved bymaking the surface impedance along the tube anisotropic and reactive, asdescribed previously. This is an extra cost and was not found to benecessery for the alternative in FIG. 6. The reactive and anisotropicsurface impedance of the subreflector is realized by means of circularcorrugations 17. These prevent the z-mode radiating strongly in a radialdirection. The excition of the φ-mode and the z-mode are controlled byvarying the dimensions of x, y, z and 2a in FIG. 7. The best results areobtained if the external part of the tube forms a waveguide whith alarger diameter than the waveguide 15, enabling both the TE₁₁ and the TM₁₁ modes to be propagated here. The resulting radiation pattern from thefeed antenna has low cross-polarization. Unfortunately there areconsiderable phase errors because the source of radiation, the aperture16, is a long way from the axis. These phase errors can be compensatedfor by shaping the main reflector differently rather than as a parabolicsurface. If the diameter of the tube is about 1λ, the optimal reflectorshape will deviate by up to 1.6 mm from the best fitted parabola. Theresultant radiation characteristics of the whole antenna are excellentand have low cross-polarization.

FIG. 6 shows one design of the antenna, it should nevertheless beapparant from the claims that there are numerous other forms possible.Common for all is that the part of the subreflector's surface which isoutside the aperture 16 has an anisotropic and reactive surfaceimpedance. Other common features are that the geometries of the centralpart 20 of the subreflector 13 and the dielectric element 21 filling thegap 14 are designed so that the required modes are excited with thecorrect phase and amplitude.

This design makes particular allowance for how the modes radiate bothalong the tube and the surface of the subreflector. The ideal shape iswhen the radiation patterns from both modes are intergrated in anoptimal manner so that the resultant pattern is in rotational symmetryand has low cross-polarization. Altering the shape of the gap 14 orfilling this completely or partially with a dielectric, are two means ofinfluencing the relative excitation of the modes.

The different elements that are illustrated in FIGS. 2 and 3 can becombined and modified in various ways. The tube 12 can be a polygonal orsquare cylinder. The subreflector can be manufactured of plastic with ametallic surface coating. The plug 21 in the gap 14 can be combined withthe subreflector 13 in other ways than those shown, for instance justone of elements 22 or 23 are used. If only element 22 is used, thesubreflector will not have a central outlet at its point 20. If onlyelement 23 is used, the subreflector will not have any corrugationsinside the aperture 16.

I claim:
 1. In an antenna system, a reflector and a feed element forradiating and intercepting electromagnetic waves, comprising:(a) a mainreflector, and (b) a self-supported waveguide feed element located alongthe axis which passes through the center of said main reflector, saidfeed element including;(1) a support-tube which has one end attached tothe center of said main reflector and the other end located near thefocal region of the reflector; (2) a waveguide located inside said tube;(3) a subreflector located outside the outer end of said tube and saidwaveguide, said subreflector having a diameter larger than saidsupport-tube; (4) a gap provided between said subreflector and the outerend of said tube, being externally bounded by an imaginary cylindricalaperture surface which has substantially the same diameter as the outerdiameter of said tube; (5) the part of said tube which is nearest to thegap having an outer surface which is mainly cylindrical with a circularcross-section; and (6) the part of the surface of said subreflectorwhich lies outside said aperture surface is planar and has ananisotropic and reactive surface impedance, and the part of saidsubreflector which lies within said aperture surface is shaped as aconverging element which has reflecting characteristics and which isinclined towards said tube.
 2. The reflector antenna system as claimedin claim 1, wherein said main reflector is rotationally symmetrical andhas a substantial parabolic shape when said tube has a diameter which issmaller than 1.0 wavelengths.
 3. The reflector antenna system claimed inclaim 1, wherein the anisotropic and reactive surface impedance of saidsubreflector is obtained by rotationally symmetrical grooves in anelectrically conducting surface.
 4. The reflector antenna system claimedin claim 1, wherein said tube has a reflecting outer surface with asubstantially anisotropic and reactive surface impedance, said impedancebeing created by circumferential corrugations.
 5. The reflector antennasystem claimed in claim 11, wherein said converging element of thesubreflector is integrated with the rest of the subreflector.
 6. Thereflector antenna system claimed in claim 11, wherein the gap betweensaid tube and said subreflector is substantially filled with adielectric element which is interlocked with said waveguide and saidsubreflector.
 7. The reflector antenna system which is claimed in claim1 wherein said waveguide has a section with a larger diameter near theouter end than that section thereof remaining in the tube.
 8. Thereflector antenna system which is claimed in claim 1 wherein thewaveguide is formed by the inner surface of the support tube.
 9. Thereflector antenna system which is claimed in claim 1, wherein thesurface impedance of said subreflector is obtained by symmetricalcorrugation formed in an electrically conducting surface.
 10. In anantenna system, a reflector and a feed element for radiating andintercepting electromagnetic waves, comprising:(a) a main reflector; and(b) a self-supported waveguide feed element located along the axis whichpasses through the center of said main reflector, said feed elementincluding,(1) a support-tube which has one end attached to the center ofsaid main reflector and the other outer end located near the focalregion of the reflector; (2) a waveguide located inside said tube; (3) asubreflector located outside the outer end of said tube and saidwaveguide; (4) a gap provided between said subreflector and the outerend of said tube, and being externally bounded by an imaginarycylindrical aperture surface, which has substantially the same diameteras the outer diameter of said tube; (5) a part of the surface of saidsubreflector which lies outside said aperture surface having ananisotropic and reactive impedance; and (6) said support tube having areflecting outer surface with a substantially anisotropic and reactivesurface impedance, said impedance being created by longitudinalcorrugations filled with a dielectric material, wherein longitudinalrefers to the length of the support-tube.
 11. An antenna system, areflector and a feed element for radiating and interceptingelectromagnetic waves comprising:(a) a main reflector; and (b) aself-supported waveguide feed element located along the axis whichpasses through the center of said main reflector, said feed elementincluding:(1) a support-tube which has one end attached to the center ofsaid main reflector and the other outer end located near the focalregion of the reflector; (2) a waveguide located inside said tube; (3) asubreflector located outside the outer end of said tube and saidwaveguide; (4) a gap provided between said subreflector and the outerend of said tube, and being externally bounded by an imaginarycylindrical aperture surface, which has substantially the same diameteras the outer diameter of said tube; (5) a part of the surface of saidsubreflector which lies outside said aperture surface having ananisotropic and reactive impedance; and (6) the part of saidsubreflector which lies within said aperture surface is shaped as aconverging element which has reflecting characteristics and which isinclined toward said tube, said converging element of the subreflectoris a separate element mounted in a central opening provided in thesubreflector.
 12. An antenna system, a reflector and a feed element forradiating and intercepting electromagnetic waves comprising:(a) a mainreflector; and (b) a self-supported waveguide feed element located alongthe axis which passes through the center of said main reflector, saidfeed element including:(1) a support-tube which has one end attached tothe center of said main reflector and the other outer end located nearthe focal region of the reflector; (2) a waveguide located inside saidtube; (3) a subreflector located outside the outer end of said tube andsaid waveguide; (4) a gap provided between said subreflector and theouter end of said tube, and being externally bounded by an imaginarycylindrical aperture surface, which has substantially the same diameteras the outer diameter of said tube; (5) a part of the surface of saidsubreflector which lies outside said aperture surface having ananisotropic and reactive impedance; (6) the part of said subreflectorwhich lies within said aperture surface is shaped as a convergingelement which has reflecting characteristics and which is inclinedtowards said tube; and (7) the gap between said tube and saidsubreflector is substantially filled with a dielectric element which isinterlocked with said waveguide and said subreflector, and saiddielectric element has a central pin pointing towards and connected to acorresponding outlet in said converging element.
 13. An antenna system,a reflector and a feed element for radiating and interceptingelectromagnetic waves comprising:(a) a main reflector; and (b) aself-supported waveguide feed element located along the axis whichpasses through the center of said main reflector, said feed elementincluding:(1) a support-tube which has one end attached to the center ofsaid main reflector and the other outer end located near the focalregion of the reflector; (2) a waveguide located inside said tube; (3) asubreflector located outside the outer end of said tube and saidwaveguide; (4) a gap provided between said subreflector and the outerend of said tube, and being externally bounded by an imaginarycylindrical aperture surface, which has substantially the same diameteras the outer diameter of said tube; (5) a part of the surface of saidsubreflector which lies outside said aperture surface having ananisotropic and reactive impedance; (6) the part of said subreflectorwhich lies within said aperture surface is shaped as a convergingelement which has reflecting characteristics and which is inclinedtowards said tube; and (7) the gap between said tube and saidsubreflector is substantially filled with a dielectric element which isinterlocked with said waveguide and said refletor, and said dielectricelement has a circular protrusion which is interlocked with a circulargroove in said subreflector.